1. Field of the Invention
The present invention relates to an internal combustion engine specifically including a crank chamber having a closed structure.
2. Related Art
In general, an engine includes a piston, and for a conventional four stroke engine, when a piston moves from the bottom dead center to the top dead center in a cylinder, a negative pressure is produced in the closed crank chamber to prevent the lifting of the piston. Against such defective matter, a conventional technology provided a solution, for example, disclosed in Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 61-182406), which discloses an internal combustion engine, in which a balancing hole is formed in a side wall of a cylinder so as to communicate an interior of the crank chamber with an exterior thereof, thereby preventing an increasing of a negative pressure in the crank chamber.
However, the internal combustion engine described in Patent Document 1 has such a disadvantage as that the balancing hole is closed when the piston is positioned at a bottom position of the cylinder, and in such occurrence, the negative pressure in the crank chamber cannot be effectively prevented until the balancing hole has been opened even if the piston is being lifted.
Further, it may be considered, for preventing the balancing hole from being closed by the movement of the piston, to form the balancing hole in the crank chamber. However, in a certain case, the engine lubricant oil used for the internal combustion engine may be contained with abrasion particles produced by abrasion of the cylinder or other components, and the abrasion particles may block the balancing hole. According to this reason, it is not preferred to form the balancing hole in a range near a surface of the lubricant oil or in an oil reaching range of the lubricant oil sprayed from the rotating crank shaft.